Transdisciplinary research practice has become a core element of global sustainability science. Transdisciplinary research brings with it an expectation that people with different backgrounds and interests will learn together through collective problem solving and innovation. Here we introduce the concept of “transdisciplinary communities of practice, ” and draw on both situated learning theory and transdisciplinary practice to identify three key lessons for people working in, managing, or funding such groups. (1) Opportunities need to be purposefully created for outsiders to observe activities in the core group. (2) Communities of practice cannot be artificially created, but they can be nurtured. (3) Power matters in transdisciplinary communities of practice. These insights challenge thinking about how groups of people come together in pursuit of transdisciplinary outcomes, and call for greater attention to be paid to the social processes of learning that are at the heart of our aspirations for global sustainability science.